1. Field of the Invention
The present invention relates to an edge-type surface light equipment for use in a rear projection liquid crystal display or sign board, or the like as the backlight thereof, and to a light guide plate, a component part of the surface light equipment. More particularly, the present invention relates to a surface light equipment which has narrow emergent light distributions both in the longitudinal direction and in the lateral direction, which shows only a small light quantity loss, which has a uniform surface luminance distribution and which is composed of only a small number of parts, and to a light guide plate useful for the surface light equipment.
2. Background Art
Conventional backlights for use in rear projection displays such as liquid crystal displays are classified into two types, the edge type and the direct type, depending upon the position of the light sources in the displays. In liquid crystal displays, the edge-type backlights are usually used because they are advantageous in saving space and in producing thin displays. A conventional edge-type surface light equipment has, for example, a structure as shown in a cross-sectional view in FIG. 10. In this figure, reference numeral 91 indicates a light guide plate, reference numeral 92 indicates a linear light source, reference numeral 93 indicates a light reflector such as a light-reflective film, reference numeral 94 indicates a light-diffusive film, and reference numeral 95 indicates a lens film. The conventional light guide plate 91 employs a light guide plate base 96 which is obtained by cutting a base made from a transparent resin such as an acrylic resin, and polishing the surface of the cut out base, or by injection-molding a transparent resin into a predetermined shape; and a light-diffusive/reflective part 97 is formed on the back surface of this light guide plate base 96. Further, a cold cathode fluorescent tube is used as the linear light source 92; a metal-deposited or whitened resin film is used as the light reflector 93; a milky white resin film is used as the light-diffusive film 94; and a resin film provided with a large number of triangular prisms is used as the lens film 95. In order to provide the light-diffusive/reflective part 97 on the back surface of the light guide plate 91, a large number of fine dots are printed by using a white ink on the back surface of the light guide plate base 96. The density of these dots is so controlled that the dots become more dense as the distance from the light source 92 becomes longer (on the right side in the figure), thereby making up the insufficiency of the quantity of emergent light on the side opposite to the light source 92 to make the surface luminance distribution uniform. Furthermore, light emerged from the light guide plate 91 is scattered by the light-diffusive film 94. The light source and the printed dots are thus prevented from being directly seen by the emergent light.
However, in the conventional surface light equipment as described above, the light-diffusive/reflective part provided on the back surface of the light guide plate is composed of a large number of fine dots formed by screen printing. Therefore, if the screen mesh of a printing plate is clogged while screen printing is conducted, dots having a desired shape and area cannot be obtained. Thus, the yield becomes low. In addition, a light-diffusive film is required to make the printed dots invisible. This increases the number of parts needed for producing the surface light equipment. The production process is thus complicated due to the dot printing, the incorporation of a light-diffusive film, etc., and the production cost is therefore inevitably increased.
Further, depending upon the application of a surface light equipment, there is also such a display that it is enough that an image on the display can only be seen from directions in a specific narrow range (e.g., from the direction of the normal). For such an application in which sufficiently high brightness is required only within a narrow range of viewing angle, the above-described equipment having the conventional structure is not suitable because it scatters emergent light too widely.